Galaxy killer

The vast expanse of the universe resembles an expansive ocean with scattered islands of galaxies, creating an illusion of emptiness. However, this perception is deceptive, as only a small portion of all atoms can be found within galaxies, while the majority is believed to drift in the intergalactic medium. Similar to the spreading roots of a colossal tree, gas emanates from each galaxy, guided by gravity, supplying fresh mass to this dense cosmic forest. Within the intergalactic medium lie the fundamental building blocks of creation: hydrogen and helium, intricately woven into sheets and filaments that flow into galaxies, eventually giving birth to stars.

Nevertheless, upon closer inspection, the true architects of the universe reveal themselves: Quasars, the most dominant entities in existence. Though minuscule compared to the grandeur of the cosmos, these quasars reside at the hearts of certain galaxies, radiating with the luminosity of a trillion stars. They unleash immense jets of matter, reshaping the surrounding cosmos entirely. Their power is so overwhelming that they possess the ability to extinguish entire galaxies.

What exactly are these enigmatic objects, and how do they mold the structure of the universe according to their desires? Everywhere one looks in the sky, peculiar phenomena abound. In the 1950s, astronomers detected mysterious and resounding radio waves emanating from various locations in the sky. Dubbed "quasi-stellar radio sources," or quasars, these celestial objects appeared as dots akin to stars, but they were visible in radio waves rather than the conventional visible light spectrum.

Quasars proved to be utterly peculiar in every aspect. Some exhibited flickering behavior, while others emitted high-energy X-rays alongside radio waves. However, they all shared a common attribute: their diminutive size. Furthermore, they moved at extraordinary speeds, surpassing 30% of the speed of light. The only plausible explanation was that these objects were so distant that their apparent velocity was a result of the universe's expansion, causing them to move away from us.

However, these colossal distances indicated that quasars were not mere stars but rather the active cores of galaxies billions of light-years away. Astonishingly, they outshone the entirety of our Milky Way galaxy by thousands of times, radiating with unfathomable intensity and unleashing radiation and light with unprecedented vigor.

As our exploration of the cosmos unfolded, we discovered over a million quasars, all of which appeared to reside in the farthest reaches of space. Considering that distant also implies looking into the past, their light took billions of years to reach us, making them relics from the early universe when both galaxies and the universe itself were in their infancy.

Let us journey back in time, approximately 3 billion years after the colossal explosion that marked the beginning of our universe, and observe the state of affairs during that era. The extraordinary brilliance and violence exhibited by quasars puzzled astronomers. How could a young, developing galaxy radiate with such immense luminosity?

It became evident that the source of this astronomical energy could not be attributed to stars alone, as their numbers were insufficient. Moreover, given the gradual growth of galaxies through merging processes over time, the starlight emitted by smaller galaxies should not have exceeded the brightness of present-day galaxies by such a wide margin.

There was only one conceivable way to generate the colossal energy radiated by quasars: by feeding supermassive black holes. Although the precise formation mechanisms of these black holes remain unknown, it appears that every galaxy harbors one at its core.

The concept of the brightest objects in the universe being black holes, notorious for capturing anything that ventures into their event horizons, may seem paradoxical. However, the illumination of quasars does not originate from within these black holes but from the surrounding space—a vast accretion disk composed of orbiting gas. Quasars employ matter as their fuel, just like stars, except black holes are the most efficient engines for converting matter into energy throughout the universe. The energy released by matter falling into a black hole can be up to 60 times greater than that generated by nuclear fusion within a star's core since a black hole's energy stems from gravity rather than nuclear reactions.

As matter plunges toward a black hole, it accelerates to nearly the speed of light before crossing the event horizon, carrying an immense amount of kinetic energy. Of course, once inside the black hole, this energy becomes concealed. One can only observe this energy if matter is dropped into the black hole in a specific manner. When a substantial amount of matter is present, it spirals rapidly toward the event horizon, forming a disk. Collisions between particles within this confined space generate friction and heat, heating the disk to hundreds of thousands of degrees. Within a region barely larger than our solar system, the core of a galaxy can radiate energy many times greater than the combined luminosity of all its stars.

This is the essence of a quasar—a supermassive black hole indulging in a cosmic feast. And these voracious black holes possess insatiable appetites, devouring an astonishing amount of gas, ranging from one to a hundred times the mass of Earth per minute. Ten billion years ago, when the universe was approximately one-third of its current size, the intergalactic medium was far less dispersed. Consequently, the filaments of gas surrounding quasars provided an abundant banquet, compelling them to spew out unimaginable amounts of light and radiation.

The most luminous quasars generate jets of energy that intertwine with the magnetic field of the matter around them, forming narrow cones. These enormous beams of matter resemble particle accelerators, carving their way through the circumgalactic medium, creating plumes of material that extend for hundreds of thousands of light-years. The sheer scale of this phenomenon is nearly inconceivable—a minute region within a galaxy shaping patches of the universe measuring hundreds of thousands of light-years in length.

However, quasars cannot sustain their gluttonous feasting indefinitely, lasting only a few million years, as their indulgence ultimately spells doom for their host galaxies. Well, perhaps "killing" is too extreme a term, as the galaxy remains intact even after the quasar dissipates. Nevertheless, it undergoes irrevocable transformations. The scorching temperatures generated by quasars cause excessive heating, stifling the formation of new stars.

Hot gas is incapable of fostering star formation. This may seem counterintuitive, considering that stars emerge from the gravitational collapse of gas that subsequently heats up. However, within a hot gas cloud, atoms move at high velocities. Consequently, collisions between atoms occur with great force, exerting pressure that resists the gravitational collapse necessary for star formation. In contrast, cold gas represents the ideal precursor for star formation since it lacks the resistance exhibited by heated gas when collapsing into a star.

Moreover, quasars expel gas from their galaxies, depriving both themselves and their host galaxies of the raw materials required for star formation. While this may sound disheartening, it may, in fact, be fortuitous for the existence of life. The alternative scenario—excessive star formation—can be significantly more perilous, as it is invariably followed by the cataclysmic explosions of massive stars in supernovae events, potentially eradicating any inhabitable planets.

Nonetheless, the intricate interplay within a galaxy's ecosystem involves various factors influencing and depending on one another. Although hot phenomena like quasars and supernovae tend to expel gas from galaxies, shockwaves and quasar jets can also compress gas, temporarily triggering new star formation. Furthermore, gas expelled from a galaxy mixes with inflowing gas, eventually recycling back into the galactic environment. Overall, it is reasonable to assume that, without certain processes cooling things down, life as we know it would not exist today.

This brings us to the final question: Did the Milky Way experience a quasar phase in the past? Regrettably, our knowledge of its ancient history remains uncertain. Galaxies have a propensity for obfuscating their past, much like sand on a beach continuously mixing away traces of its origins. While it is plausible that the Milky Way once harbored a quasar, potentially contributing to the growth of our supermassive black hole, Sagittarius A*, to a mass 4 million times that of the Sun, concrete evidence eludes us.

Although currently dormant, Sagittarius A* has the potential to transform into a quasar in the future. In a few billion years, the Milky Way is set to merge with the Andromeda galaxy. Over a hundred "double quasars" resulting from galaxy collisions have been observed, providing fresh gas to fuel the central black holes. However, this phenomenon will not endure. When galaxies merge, their supermassive black holes also converge, sinking into the center of the newly formed galaxy and stirring up dust and stars in all directions.

While we cannot definitively predict whether such an event will occur, it would undoubtedly be a breathtaking spectacle. Perhaps beings in the distant future will have the privilege of witnessing this awe-inspiring phenomenon. Nevertheless, we need not wait that long, as there are already countless fascinating phenomena to explore right here on our planet, in the present—provided we possess the knowledge to comprehend them.

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